This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-341100, filed on Nov. 8, 2000; the entire contents of which are incorporated herein by reference.
This invention is related to a current transducer (CT) system for measuring high-voltage main circuit AC current in a gas-insulated switchgear (GIS) for AC power supply system, the switchgear with the current transducer system and a method for measuring the AC current.
In general air-insulated substations, switchgears are sometimes replaced by new ones, while busbars and steel towers are rarely replaced since their life spans are relatively long. Therefore, in most replacement works, the switchgears are replaced by new ones of higher performance and higher reliability utilizing installed older air-insulated busbars. For those purposes, some integrated gas-insulated switchgears have been proposed.
Referring to FIGS. 1 and 2, typical line bay construction of an air-insulated substation of double busbar scheme of the prior art is described now. The air-insulated substation has a circuit breaker (CB) 1 which is connected to busbars 101a and 101b via bus side disconnecting switches (DSs) 2a and 2b, respectively. The substation also has a pair of current transformers 102a and 102b on both sides of the circuit breaker 1. The substation also has a line side disconnecting switch 2c, an instrument transformer (or a voltage transformer: VT) 103 and a lightning arrester (LA) 104.
Those components of the substation are supported on insulating supporting structures. The output AC current is sent out of the substation through over-head lines (OHLs) 130. The current transformers 102a and 102b are typically of oil-filled insulator types.
FIG. 3 shows an exemplary construction of components of a conventional substation replaced with new integrated gas-insulated switchgears and conventional current transformers. The component groups surrounded by dot-dash lines 105 and 106 shown in FIG. 1 might be replaced by integrated gas-insulated switchgears 107 and 108, respectively, as shown in FIG. 3.
The component group surrounded by a dot-dash line 109 shown in FIG. 3 is now described referring to FIG. 4. The integrated gas-insulated switchgear 107 may have a metal box 13a containing a driving connector 121e. The metal box 13a may be supported by and disposed on top end of a vertically standing supporting hollow insulator 11a. 
An operation device 12a may be disposed at the bottom end of the supporting hollow insulator 11a. Hollow insulators 14e and 14f may be supported by the metal box 13a on their ends and contain the disconnecting switches 2a and 2b, respectively. The disconnecting switches 2a and 2b may be operable by the operation device 12a via the driving connector 121e. 
Likewise, the integrated gas-insulated switchgear 108 may have a metal box 13b containing a driving connector 121. The metal box 13b may be supported by and disposed on top end of a vertically standing supporting hollow insulator 11b. An operation device 12b may be disposed at the bottom end of the supporting hollow insulator 11b. 
Hollow insulators 14a and 14c may be supported by the metal box 13b on their ends and contain the circuit breaker 1 and the disconnecting switch 2c, respectively. The circuit breaker 1 and the disconnecting switch 2c may be operable by the operation device 12b via the driving connector 121.
The disconnecting switches 2a and 2b, the circuit breaker 1 and the disconnecting switch 2c may be electrically connected each other in series in the metal boxes 13a and 13b, respectively.
Thus, the components surrounded by the dot-dash line 105 shown in FIG. 1 would be replaced by the integrated gas-insulated switchgear 107 having the two disconnecting switches 2a and 2b, and the components surrounded by the dot-dash line 106 would be replaced by the integrated gas-insulated switchgear 108 having the circuit breaker 1 and the disconnecting switch 2c as shown in FIGS. 3 and 4.
Now replacement of current transformers is discussed. The line side current transformer 102b shown in FIGS. 1 and 2 can be replaced by a penetrating-type current transducer, which is of the same type used in conventional gas-insulated switchgears, in the integrated gas-insulated switchgear if the line-side (e.g. the metal box 13b shown in FIG. 4) is grounded. However, the bus side current transformer 102a shown in FIG. 1 cannot be replaced by a penetrating-type current transducer for either of the integrated gas-insulated switchgears 107 or 108 shown in FIG. 4, since the replaced current transformer would be on the high-voltage side.
Therefore, oil-filled insulator type current transformers must be used. Thus, if the air-insulated substation shown in FIG. 1 is replaced with an integrated gas-insulated switchgear shown in FIG. 3, the current transformers used there would be a combination of current transformers of oil-filled insulator-type and of penetrating-type.
General line bay construction of air-insulated substation of double busbar scheme have been discussed, whereas combination of oil-filled insulator-type current transformers and penetration-type current transformers may have to be utilized in other facilities such as bus section or bus coupler constructions, line bay constructions of single busbar scheme substations, line bay constructions of one and half busbar scheme substations, and so on.
As discussed above, a penetration-type current transformer which is generally used in conventional gas-insulated switchgears cannot be disposed at the high-voltage side for the integrated gas-insulated switchgear 108 shown in FIG. 4 due to the insulation difficulty, so that the circuit breaker cannot have current transformers on its both sides. Therefore, in practice, disposing current transformer on one side of the circuit breaker may be omitted, or an independent oil-filled insulator-type current transformer may be installed if current transformers on both sides of the circuit breaker would be necessary.
Oil-filled insulator-type current transformers themselves are expensive, and would require extra area and ground construction for setting up, which would spoil the advantage of reduced required area for substations and the total cost-down which would be generally obtained by applying integrated gas-insulated switchgears.
In addition, in an oil-filled insulator-type current transformers, internal burnout accident could cause increased pressure in the tank which could develop into a severe accident with a tank explosion. Therefore, the oil-filled insulator-type current transducers would be demanded to be replaced by penetrating-type current transformers when the substations are replaced.
However, the penetrating-type current transformers used in conventional gas-insulated switchgears with analog current output, the rated secondary output of which may be 1 Ampere or 5 Amperes, can be installed only in ground voltage side, and cannot be disposed on both sides of the circuit breaker of integrated gas-insulated switchgears such as the ones shown in FIG. 3.
Accordingly, it is an object of the present invention to provide an improved current transformer which can be disposed on either side of the circuit breaker of an integrated gas-insulated switchgear.
There has been provided, in accordance with an aspect of the present invention, a current transducer system for measuring AC current in a high-voltage main circuit having a circuit breaker and a disconnecting switch in an integrated gas-insulated switchgear, the system comprising: a current sensor for detecting the AC current and outputting an analog electric signal representing the AC current, the current sensor disposed near the circuit breaker; a sensor unit including an analog-to-digital converter for converting the analog electric signal to a digital electric signal, and an electric-to-optic converter for converting the digital electric signal to a digital optic signal, the sensor unit disposed near the circuit breaker; and optic transmission means for transmitting the digital optic signal.
There has also been provided, in accordance with another aspect of the present invention, an integrated gas-insulated switchgear for switching on and off an AC current in a high-voltage main circuit, the switchgear comprising: (1) a container filled with insulating gas; (2) a circuit breaker and a disconnecting switch contained in the container, the circuit breaker and the disconnecting switch being connected in series each other; and (3) a current transducer system for measuring the AC current in the high-voltage main circuit, the current transducer system including: a current sensor for detecting the AC current and outputting an analog electric signal representing the AC current, the current sensor disposed near the circuit breaker; a sensor unit including an analog-to-digital converter for converting the analog electric signal to a digital electric signal, and an electric-to-optic converter for converting the digital electric signal to a digital optic signal, the sensor unit disposed near the circuit breaker; and optic transmission means for transmitting the digital optic signal.
There has also been provided, in accordance with yet another aspect of the present invention, a method for measuring AC current in a high-voltage main circuit having a circuit breaker and a disconnecting switch in an integrated gas-insulated switchgear, the method comprising steps of: detecting the AC current and outputting an analog electric signal representing the AC current near the circuit breaker; converting the analog electric signal to a digital electric signal and converting the digital electric signal to a digital optic signal near the circuit breaker; and transmitting the digital optic signal.
There has also been provided, in accordance with yet another aspect of the present invention, a current transducer system for measuring AC current in a high-voltage main circuit having a circuit breaker and a disconnecting switch in an integrated gas-insulated switchgear, the system comprising: a current transducer for detecting the AC current and outputting an optic signal representing the AC current, the current transducer disposed near the circuit breaker; and optic transmission means for transmitting the optic signal.